Generic optical attenuation units or attenuators are used in particular in electronic distance measuring modules (EDM). The distance measuring modules are assemblies for example of theodolites, of profilers or of laser scanners.
Known attenuators from the prior art substantially consist of a neutral filter wheel driven by motor. In general, ten to fifty attenuation stages are moved to discretely; the attenuation can typically also be set continuously, however.
In general, such an attenuator is situated in the optical transmission channel of an EDM, but can also be used in the reception channel of the EDM. One of the particular challenges for using an attenuator in an EDM is a very large setting range from very low to extremely high optical density of at least 5.0 (=105), typically associated with an exponential profile of the transmission between these two extreme states. The exponential profile means that in an angular increment the attenuation decreases or increases by a multiplicative rather than additive factor.
The high required signal dynamic range of an EDM is attributable firstly to the large distance range to be determined and covered of from less than 1 m to more than several kilometers, and secondly to the fact that the intention is to measure highly diffusely light-scattering target objects with markedly little backscattering and also prismatic target objects with retroreflection and extremely high optical quality, and correspondingly highly intensive reflection. In this case, the signal range extends over five orders of magnitude (105), thus forming the basis for the abovementioned requirements of the high variability of the transmission.
Attenuators in widespread use at the present time are often produced from a high density photographic black/white film. Both films comprising panchromatic emulsions and films comprising orthochromatic emulsions are suitable for this purpose. Such films have a polyester carrier, and so they are lightweight, of small dimensions, simple to produce and therefore cost-effective.
In conjunction with the rapid advance of digital photography, the traditional film business has largely been superseded in the meantime. The major manufacturers both of photographic emulsions and indeed also of the carrier materials that are of entirely satisfactory optical quality are gradually stopping production and sale of such films. In particular, the production of monochrome emulsions having high optical density is affected by this current development.
Although films of a high quality level are still currently available, they only have an average optical density of less than 3. The commercially available films that remain can no longer realize the required grayscale ranges that were achieved with ortho-films. The current color or b/w films only achieve values of up to approximately OD 2.5. Even with hard working developer solutions, full grayscale value reproduction, i.e. attenuation as great as required previously, is not achievable.
One possible form of realization for setting the signal range over five orders of magnitude (105) could be realized with a double pack of films having OD 2.5. However, this would have the disadvantage of increased scattered light, which should be prevented, as known. A further disadvantage would be the longer switching times owing to an increased moment of inertia.
Some variants of attenuation filters have a line- or grid-like grating structure, wherein a monotonic increase in the attenuation is achieved by an increasing density of the lines or by reduction of the size of the free openings of the grids. In addition, the attenuation is intensified by optical diffraction. A line or grating structure can additionally intensify the optical attenuation by diffraction, and attenuation ratios that are higher than the characteristic density curve of a film can be achieved.—Attenuators having a grating-like structure are not suitable, however, in a transmission channel having a light source, such as a laser, which emits diffraction-limited light, since the orders of diffraction produce discrete beam directions that disturb the sensor function of an EDM.
With regard to electro-optical attenuators or spatial modulators, a plurality of techniques have been tried out in the last twenty years or so. In addition to devices based on liquid crystals, for example, magneto-optical, semiconductor-based multi quantum well arrangements or deformable mirrors have been proposed for use as attenuators. Hitherto, only arrangements based on liquid crystal technology (manufacturer: Boulder Nonlinear Systems) and MEMS-based micromirror technology (manufacturer: Texas Instruments) have found a use in commercial products; such an arrangement for an attenuator is also disclosed in US 2012/0224164 A1.
Non-optical signal attenuations are also conceivable, for example signal attenuation in the electrical part of the signal path. In this case, the driving of the light emitting laser diode, for the variation of the generated output light intensity, control of the APD gain of the receiving diode and also the driving of the electrical amplifier stages as far as the analog-to-digital converter and also electro-optical fiber attenuators come into consideration.
These attenuation possibilities have actually always been used, although generally only in a supplementary rather than sole function for signal attenuation; this is because the signal dynamic range that is to be covered with a distance measuring module exceeds the possibilities of these electrical attenuation methods, even if the latter are combined. By way of example, the signal from reflective articles is at least millions of times stronger than that from a dark diffuse object surface. Electrical receiving circuits of the abovementioned type achieve dynamic ranges in the range of approximately three orders of magnitude.
Optical attenuators are linear with regard to technical signal transmission behavior, that is to say that they are linear both in terms of phase (propagation time) and in terms of amplitude, and the transmitted signals are undistorted, which is advantageous for a high measurement accuracy of an EDM module.
Fiber-optic attenuation devices are also known. Although they operate rapidly and can set a desired signal strength even with a switching time of a few nanoseconds, they are inexpediently large in terms of the mechanical dimensions and relatively expensive.
However, none of the previous products based on the alternative solutions proposed to date satisfies the requirements of the high dynamic range of OD0 to OD5 or even OD6.